The aim of this effort is to evaluate the prevalence, associated conditions, and prognosis of febrile seizures, which is the most common convulsive disorder in any age group. Knowledge of the risks associated with febrile seizures is necessary for rational therapeutic decision-making. The Collaborative Perinatal Project presents a unique opportunity to evaluate, among other factors, the neurological status of children prior to the onset of any seizure, as well as other clinical characteristics, in a large, prospectively identified population. The initial report in this study has dealt with the likelihood of development of chronic epilepsy in children who have experienced febrile seizures. Work now in progress concerns the occurrence of later mental retardation, recurrence of febrile seizures, and learning and behavior disorders in the large group of children. Neurological and developmental status prior to the first febrile seizure is found to be an important determinant of outcome.